Peter - I really dont think a travel organisations guide is much to go by, their lack of correct English language in the guide does not fill me with confidence either. Their list leaves out lots of highly regarded schools.
I would try searching in the good schools guide, Tatler (for a laugh and toungue in cheek report) and the major British News papers. Then when you have found a few schools you are interested in, search for comments on them here on this forum and go for a visit.
Here are some things to think of: Full boarding or weekly (full more sensible if you are abroad), single sex or mixed, location (to airport/ relatives) sporty (which sport)
Good luck.